Structural determination of ionic liquids with theoretical methods: C 8mimBr and C8mimCl. Strength and weakness of current force fields

In this work we report the first X-ray scattering study of imidazolium-based ionic liquids containing the bromide anion. The system studied was 1-octyl-3-methyl-imidazolium bromide ([C8mim]Br). The study was extended to the analogous salt, containing chloride as anion ([C 8mim]Cl) which has been used for comparison. The measured diffraction patterns are compared with the theoretical spectra calculated from model geometries obtained with classical molecular dynamics simulations. The behavior and the performance of the available force fields in the description of bromide ion are discussed. © 2010 American Chemical Society.